The addition of 4 ug/ml of protamine to blood samples obtained by venipuncture increased PT and APTT by only 9%. Protamine supplementation of blood samples drawn through heparinized catheters reduced false elevations in PT from 44% to 12% and in APTT from 52% to 4%. No falsely normal PT or APTT tests were seen when protamine was added to samples from four patients with known coagulopathy (with and without added heparin).